The present invention relates generally to pressure sensing units and particularly to the packaging of pressure transducers for incorporation into pressure sensors or pressure sensing units so as to withstand very large shock and vibration amplitudes without sustaining damage.
Pressure sensing units that employ implanted semiconductor strain resistors are widely used in many applications. The strain-sensitive resistors are arranged on a silicon die and typically interconnected so as to form a full Wheatstone bridge on one surface. The underside of the die is usually etched to form a thin diaphragm. The edges of the diaphragm correspond to the location of the strain-sensitive resistors, or piezoresistors, where strain is the greatest when the diaphragm undergoes bending as a result of pressure loading on one side. The construction of the diaphragm and placement of the strain-sensitive resistors is dependent on the application and the manufacturer. Circular diaphragms are sometimes used where two resistors in opposite legs of the bridge are positioned radially to the diaphragm and the remaining two resistors in opposite legs of the bridge are positioned tangentially to the diaphragm. In this way the resistors positioned radially will increase resistance with pressure loading and the resistors positioned tangentially will decrease resistance. In high-pressure applications an elongated rectangular diaphragm is more likely to be used. U.S. Pat. Nos. 5,485,753 and 5,295,395, which are assigned to Honeywell Inc., describe a form of pressure sensor diaphragm and method of making that allows for the formation of long rectangular plate structures in semiconducting material, and are hereby incorporated by reference.
A package must be provided for the silicon die and package induced strains contribute to noncompensatable errors such as thermal hysteresis and drift through common adhesive materials like epoxies, which have plastic and creep characteristics. In some pressure measurement applications, these errors cannot be tolerated. This is especially true where high accuracy is required at high temperatures where the errors are more pronounced. A variety of packaging approaches have been used. In one well-known configuration, a pressure sensitive chip or die is attached to a support structure such as a Pyrex(trademark) tube. The combined structure is referred to as a chip-tube, even though the support structure may not be tubular in shape, and may be a rod, a backplate or some other structure. To obtain high performance, the length of the Pyrex(trademark) tube is typically a few tenths of an inch or more in order to separate the die from the attachment of the chip-tube to the remainder of the sensor package. This attachment of the chip-tube to another package material is subject to strains produced, for example, by differences in the temperature coefficient of expansion (TCE) of the chip-tube material and the TCE of the package material. Temperature variations then can cause strains at the junction of the chip-tube and the package and the strains can cause strains at the sensor which introduces errors. A longer chip-tube reduces the effect of these strains. Typical methods of attaching the chip-tube to the rest of the package include epoxy and solder.
The chip-tube may be in the form of a backplate of an elastic material having a (TCE) closely matched to that of silicon which is used to mount the silicon die. In order to reduce the effects of thermal hysteresis and sensor sensitivity drift, the backplate is made of a size so that the hysteresis-prone materials are small by comparison and so have a small strain transfer effect on the larger and comparatively stiffer backplate. The backplate material is usually Pyrex(trademark), but it can be silicon, SD2 glass, ceramic or other materials. The use of these materials entails tradeoffs between performance and the available packaging space and, to a degree, cost. However, no matter how large the elastic backplate, all strains cannot be eliminated and the problem is exacerbated at higher temperatures. The backplate must then be secured to another package material that is different from the backplate material.
New application needs continue to emerge for pressure sensing devices that are able to survive extreme environments of temperature and pressure. For example, the high temperatures found in internal combustion engine testing applications and the high pressures required for down-hole oil drilling applications. In addition to the temperature and pressure requirements, there is a need for pressure sensing devices that can withstand the forces due to acceleration and shock associated with various applications related to investigating or exploring geologic formations. Fluid pressure measurements associated with geologic exploration also require pressure-sensing units that accommodate liquids containing solids while allowing transmission of the pressure of the liquid. U.S. Pat. No. 6,046,667, which is assigned to Honeywell Inc., describes a pressure transducer with porous material for media isolation and is hereby incorporated by reference. In such applications the process of placing the pressure-sensing device at the desired point of measurement would subject it to high acceleration forces. For example U.S. Pat. No. 6,028,534, issued to Schlumberger Technology Corporation, describes the use of pressure sensing devices or intelligent sensors that can be positioned within a formation of interest by any suitable means. One example provided is a hydraulically energized ram that can propel the sensor into the formation causing it to penetrate the formation to a sufficient depth to allow the sensing of formation data. An alternative means is drilling into the formation with the sensor then being positioned in the drilled hole by a sensor actuator. A further alternative means is a propellant energized onboard the drill collar which can be activated to fire the sensor with sufficient force to penetrate into the formation laterally beyond the well bore. It can be appreciated that a sensor would need to be capable of surviving the forces of acceleration and deacceleration associated with these and other methods used to place the sensor at the desired measurement location. In addition, size and shape constraints of the complete sensor that will permit deployment as described above limit the space available for the transducer and its package. For example, the constraints do not permit the use of a chip-tube of a few tenths of an inch in length.
Thus a need exists for sensing units that are capable of sensing pressure and temperature, and can survive the high acceleration forces that will be encountered in placing the devices in the desired measurement location.
The present invention solves these and other needs by providing a transducer packaging assembly for use in a sensing unit subjected to high forces of acceleration. A pressure sensitive die has a backside bonded to a large backplate. A base includes a cavity that is shaped to receive the combination of the backplate and the pressure sensitive die with the backplate spaced from the surface of the cavity. A planar cover secured to the base has a hole larger than the die but smaller than the backplate. Thin wires extend from the pressure sensitive die to electrical connections on the cover. A viscous fluid fills a space between the backplate and the cavity and a space between the backplate and the cover and transmits a pressure to be measured to the pressure sensitive die while cushioning the combination of the pressure sensitive die and backplate from forces of acceleration. A thin semipermeable material is immersed in the viscous fluid and surrounds a portion of the backplate to aid in centralizing and cushioning the backplate.